The present invention relates generally to biochemical assays, and more particularly to biochemical assays conducted through electrowetting techniques.
Typically, biochemical assays (such as those performed in drug research, DNA diagnostics, clinical diagnostics, and proteomics) are performed in small volume (50-200 xcexcL) wells. Multiple wells are ordinarily provided in well plates (often in groups of 96 or 384 wells per plate). In additional to the bulk of the wells themselves, the reaction volumes can require significant infrastructure for generating, storing and disposing of reagents and labware. Additional problems presented by typical assay performance include evaporation of reagents or test samples, the presence of air bubbles in the assay solution, lengthy incubation times, and the potential instability of reagents.
Techniques for reducing or miniaturizing bioassay volume have been proposed in order to address many of the difficulties set forth hereinabove. Two currently proposed techniques are ink jetting and electromigration in capillary channels (these include electroosmosis, electrophoresis, and combinations thereof). Ink jetting involves the dispensing of droplets of liquid through a nozzle onto a bioassay substrate. However, with ink jetting it can be difficult to dispense precise volumes of liquid, and this technique fails to provide a manner of manipulating the position of a droplet after dispensing. Electromigration involves the passage of electric current through a liquid sample. The transmission of the electric current can tend to separate ions within the solution; while for some reactions this may be desirable, for others it is not. Also, the passage of current can heat the liquid, which can cause boiling and/or the occurrence of undesirable chemical reactions therein.
An additional technique for performing very low volume bioassays that addresses at least some of the shortcomings of current techniques is electrowetting. In this process, a droplet of a polar conductive liquid (such as a polar electrolyte) is placed on a hydrophobic surface. Application of an electric potential across the liquid-solid interface reduces the contact angle between the droplet and the surface, thereby making the surface more hydrophilic. As a result, the surface tends to attract the droplet more than surrounding surfaces of the same hydrophobic material that are not subjected to an electric potential. This technique can be used to move droplets over a two-dimensional grid by selectively applying electrical potentials across adjacent surfaces. Exemplary electrowetting devices are described in detail in co-assigned and co-pending U.S. patent application Ser. No. 09/490,769, filed Jan. 24, 2000 now U.S. Pat. No. 6,565,727, the content of which is hereby incorporated herein in its entirety.
In view of the foregoing, it would be desirable to provide a technique for employing electrowetting processes that can enable a droplet to move in three-dimensions.
The present invention can enable droplets within an electrowetting device to move in three dimensions. As a first aspect, the present invention is directed to an apparatus for inducing movement of an electrolytic droplet comprising: a housing having an internal volume filled with a liquid immiscible with an electrolytic droplet; a distribution plate positioned within the chamber having an aperture therein, the distribution plate dividing the housing into upper and lower chambers; a lower electrode positioned below the lower chamber and below the aperture in the distribution plate, the lower electrode being electrically insulated from the lower chamber and being separated from the lower chamber by an overlying hydrophobic layer; an upper electrode located above the upper chamber and above the aperture of the distribution plate, the upper chamber electrode being electrically insulated from the upper chamber and being separated from the upper chamber by an underlying hydrophobic layer; and first, second and third voltage generators that are electrically connected to, respectively, the lower and upper electrodes and the distribution plate. The first, second and third second voltage generators are configured to apply electrical potentials to the lower and upper electrodes and to the distribution plate, thereby inducing movement of the electrolytic droplet between the hydrophobic layers of the upper and lower chambers.
With a device of this configuration, the device is capable of moving an electrolytic droplet outside of the two-dimensional plane typically defined by the lower chamber. As such, a droplet can be raised into contact with the hydrophobic layer of the upper chamber, which may be coated with a reactive substrate that reacts with constituents of the electrolytic droplet. Thus, reactions can be carried out in one location in the upper chamber as other droplets are free to move below the reacting droplet. Also, the upper chamber may include multiple sites of reactive substrate, which may be identical, may contain the same substrate in varied concentrations, or may contain different substrates. As such, the hydrophobic layer of the upper chamber may serve to identify and quantify constituents of the electrolytic droplet.
The device described above may be used in the following method, which is a second aspect of the present invention. The method comprises: providing a housing having an internal volume and a distribution plate residing therein, the distribution plate having an aperture and dividing the internal volume into upper and lower chambers, the lower chamber including an electrolytic droplet and each of the upper and lower chambers containing a liquid immiscible with the electrolytic droplet, the housing including a lower electrode electrically insulated from the lower chamber and underlying a hydrophobic layer, and the housing further including an upper electrode electrically insulated from the upper chamber and overlying a hydrophobic lower layer; positioning the electrolytic droplet above the lower electrode and beneath the distribution plate aperture; and applying electrical potentials to the lower and upper electrodes and to the distribution plate to draw the electrolytic droplet through the distribution plate aperture and to the upper chamber hydrophobic surface.
As a third aspect, the present invention is directed to an apparatus for inducing movement of an electrolytic droplet. The apparatus comprises: a housing having an internal volume; a plurality of adjacent, electrically isolated transport electrodes positioned in the housing, wherein sequential transport electrodes have substantially contiguous, hydrophobic surfaces, the transport electrodes defining a droplet travel path; a first voltage generator electrically connected to the transport electrodes, the first voltage generator configured to apply electrical potentials sequentially to each transport electrode along the droplet travel path, thereby inducing movement of an electrolytic droplet along the travel path; a plurality of gate electrodes, each of the gate electrodes positioned in the housing adjacent a respective transport electrode and having a hydrophobic surface that is substantially contiguous with the hydrophobic surface of the adjacent transport electrode, the gate electrodes being electrically connected; a second voltage generator connected to the plurality of gate electrodes and configured to apply electrical potentials thereto; a plurality of destination electrodes, each of which is positioned in the housing adjacent a respective gate electrode, each destination electrode having a hydrophobic surface that is substantially contiguous with the hydrophobic surface of the adjacent gate electrode; and a third voltage generator connected to the destination electrodes and configured to apply electrical potentials thereto. This configuration enables the device to xe2x80x9cparkxe2x80x9d electrolytic droplets in the destination electrodes prior to, during or after processing while allowing other droplets to use the travel path defined by the transport electrodes.